It is well recognized in modern society that hazardous waste management has become an important consideration. In response to widespread public concern, substantial amounts of money are expended for waste treatment systems so that the varied, complex and hazardous products of human activity can be effectively removed from the environment. It is preferred, of course, that these products be foreclosed from entering the environment in the first place.
Thus, it is generally acknowledged that preventing the introduction of hazardous materials into the environment has significant value. In this regard, air pollution control systems have been shown to be useful in removing unwanted airborne substances from a waste gas stream prior to its release into the atmosphere. As a rule, effective pollution control systems eliminate, or at least substantially reduce, undesirable airborne substances at or near the point of their generation. In this manner, by addressing the problem near its source, contaminants may be effectively removed when they are at their greatest concentration.
Historically, particulate contaminants issuing from smoke stacks received early attention and, for some time, flue gas scrubbers have been utilized in an attempt to reduce particulates in the waste gases from furnaces and incinerators. In the case of larger flue particulate contaminants, wet scrubbers have been utilized for treating the gas and removing the particulates.
In a typical wet scrubber, particle removal is achieved when water under pressure is sprayed into a treatment chamber through which the flue gas flows. In large measure, the removal function is mechanical rather than chemical. Large orifice nozzles, producing large droplets, are suitable because large aqueous surface areas are not required for mediating chemical reactions. Examples of conventional gas scrubbers are seen in U.S. Pat. Nos. 3,894,853 and 5,147,421.
While conventional scrubbers are useful in some applications, they are often not suitable for the more challenging removal problems presented by emissions from sources of hot gas streams. In some cases, these gas streams are produced by incinerators operating at very high temperatures for the incineration of hazardous wastes from hospitals, municipal sewage sludge and other sources. The hot flue gas contains a variety of hazardous constituents, including heavy metals, acid gases, ash and known or suspected carcinogenic substances. Conventional scrubbers alone are generally ineffective in removing these substances.
In general, the flue gas streams from incinerators carry relatively large particles having diameters in the 3-4 micron range down to smaller particles having a diameter of about 0.3 microns or less.
In cases where it has been necessary to remove particles having a diameter of less than 1 micron, Venturi scrubbers have had some value. These scrubbers are often gas atomizing devices which rely on shearing and impaction forces to break water into fine droplets to attain an even distribution of droplets at high density. As a result, there exists a probability that a contaminant particle, on impacting a droplet, will become enlarged and ultimately inertially separated. In these systems, large volumes of water and unacceptably high energy inputs are sometimes required for effective flue gas treatment. Thus, Venturi scrubbers alone are not generally suitable for removal of unwanted substances from incinerator flue gases.
Concomitant with the development of Venturi scrubbers has been development of electrostatic precipitators which, in some applications, are effective in the removal of airborne particulates. Examples of electrostatic precipitators are seen in U.S. Pat. Nos. 1,329,825; 1,479,270; 2,730,195; 3,765,154; 3,958,961 and 4,251,234.
Generally, in a high energy Venturi, the practical limits of inertial based removal techniques are not suitable for removal of submicron particles since such particles behave as if they have little or no mass. When these particulates are to be removed, conventional systems have sometimes employed the principle of electrostatic attraction so that a conventional scrubber could have its removal efficiency enhanced by the application of a positive charge to the incoming gas stream. Because water tends to be electronegative, small particulates can be attracted to the water by electrostatic forces and subsequently washed out of the system as sludge. Thus, for some applications, combinations of scrubbers and electrostatic precipitators are useful. Such combinations are disclosed in U.S. Pat. Nos. 4,019,444; 4,256,468; 4,305,909; 4,957,512 and 5,154,734.
The Venturi scrubber/electrostatic precipitator combination, while suitable in certain instances, has limited utility in incinerator flue gas treatment. Generally in such applications, these systems are expensive, requiring high energy consumption and consuming unacceptably large amounts of water and reagents. Further, the systems are often very large, requiring substantial amounts of space. Because of their size, installation, plumbing and maintenance costs are significantly increased.
In addition to the above mentioned limitations of conventional air pollution control systems, a short gas dwell time in the system sometimes presents a problem since too rapid a gas flow through a precipitator results in unacceptable system performance. The attempt to increase dwell time in conventional systems has sometimes required increasing the size of pollution control systems employing electrostatic precipitators. As a result, pollution control systems tend to become very large in the attempt to accommodate the high gas flow rates produced by modern incinerators. In spite of their size, however, many conventional systems are still unable to remove incinerator gas particulates at an acceptable level of performance and at an economical cost of operation.
The problem presented when the gas stream passes too rapidly through the electrostatic precipitator was recognized early in the development of gas treating systems and some attempts have been made to remedy the condition. In this regard, reference may be made to U.S. Pat. Nos. 1,329,825 and 1,479,270 in which an obstruction plate is located upstream of the precipitator. In U.S. Pat. Nos. 2,730,195 and 3,958,961, upstream baffles are utilized. While such obstructions serve to delay gas flow through the precipitator, they also can cause turbulence in the gas stream thereby sometimes diminishing precipitator performance. Further, they add to the expense of system operation because of the additional energy required to drive the gas through the system. Thus, the use of obstructing baffles and plates has limited value in improving system performance.
Another problem encountered in the use of wet electrostatic precipitators is that, in some cases, the fluids entering the precipitator contain unmixed streams of gas and water. This maldistribution results in diminished precipitator performance and, as a consequence, the release into the environment of undesirable substances. In addition, such maldistribution can contribute to corrosion within the precipitator, thereby shortening the useful life of the system.
In view of the foregoing, it would be highly desirable to have a pollution control system which would be efficient and economical in operation, effective in removing a broad spectrum of unwanted incinerator flue gas substances. Such a system would optimize precipitator performance while reducing deterioration of the precipitator. In addition, the system would be relatively maintenance free and would be substantially more compact than conventional systems.